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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2248) Provide new non-copy mechanism to assure atomic reads in get and scan
Date Thu, 04 Mar 2010 20:15:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841488#action_12841488
] 

Jonathan Gray commented on HBASE-2248:
--------------------------------------

Might be time to turn gets into scans so we don't have a second read code path.

> Provide new non-copy mechanism to assure atomic reads in get and scan
> ---------------------------------------------------------------------
>
>                 Key: HBASE-2248
>                 URL: https://issues.apache.org/jira/browse/HBASE-2248
>             Project: Hadoop HBase
>          Issue Type: Bug
>    Affects Versions: 0.20.3
>            Reporter: Dave Latham
>             Fix For: 0.20.4
>
>         Attachments: HBASE-2248-demonstrate-previous-impl-bugs.patch, HBASE-2248-ryan.patch,
hbase-2248.gc, HBASE-2248.patch, Screen shot 2010-02-23 at 10.33.38 AM.png, threads.txt
>
>
> HBASE-2037 introduced a new MemStoreScanner which triggers a ConcurrentSkipListMap.buildFromSorted
clone of the memstore and snapshot when starting a scan.
> After upgrading to 0.20.3, we noticed a big slowdown in our use of short scans.  Some
of our data repesent a time series.   The data is stored in time series order, MR jobs often
insert/update new data at the end of the series, and queries usually have to pick up some
or all of the series.  These are often scans of 0-100 rows at a time.  To load one page, we'll
observe about 20 such scans being triggered concurrently, and they take 2 seconds to complete.
 Doing a thread dump of a region server shows many threads in ConcurrentSkipListMap.biuldFromSorted
which traverses the entire map of key values to copy it.  

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